As is well known, a servo motor system is able to precisely control the rotation speed of a servo motor, and has the fast response to acceleration, deceleration and reversion. Due to the precise position control capability and the speed control capability, the servo motor system has been widely used in various industrial automation industries and precision machining fields such as mechanical arms or mechanical work platforms.
FIG. 1A is a schematic functional block diagram illustrating the architecture of a conventional servo motor system. As shown in FIG. 1A, the servo motor system comprises a command device 110, a micro controller 120, a servo motor 130, and an optical encoder 140. Furthermore, the optical encoder is also called a photo sensor.
In response to the user's operation, the command device 110 generates a command pulse for controlling a rotation speed and a rotation direction of the servo motor 130. Moreover, according to the rotation speed and the rotation direction of the servo motor 130, the optical encoder 140 generates a feedback pulse to the micro controller 120. Moreover, according to the command pulse and the feedback pulse, the micro controller 120 generates a driving pulse to the servo motor 130.
By the optical encoder 140, a displacement amount of a rotating shaft of the servo motor 130 is transformed into the feedback pulse. According to the feedback pulse from the optical encoder 140, the micro controller 120 may realize the rotation speed, the rotation direction and the position of the servo motor 130.
For example, the optical encoder 140 is a rotary optical encoder. The optical encoder 140 comprises a light source 142, a photo detector 146, and a disk 148. The disk 148 is coupled to the rotating shaft of the servo motor 130. In addition, the disk 148 is rotated with the servo motor 130. Moreover, after a light beam emitted by the light source 142 passes through gratings of the disk 148, the light beam is received by the photo detector 146. According to the shapes of the gratings of the disk 148, the photo detector 146 generates two photoelectronic signals A and B. According to the two photoelectronic signals A and B, the optical encoder 140 generates the feedback pulse to the micro controller 120.
FIG. 1B is a schematic timing waveform diagram illustrating the two photoelectronic signals A and B generated by the photo detector of the servo motor system of FIG. 1A. Generally, as the frequencies of the two photoelectronic signals A and B are increased, the rotation speed of the servo motor 130 is increased. In addition, there is a phase difference between the two photoelectronic signals A and B. For example, if the phase of the photoelectronic signal B leads the phase of the photoelectronic signal A by 90 degrees, the servo motor 130 is rotated in a first direction (e.g. a clockwise direction). Whereas, if the phase of the photoelectronic signal A leads the phase of the photoelectronic signal B by 90 degrees, the servo motor 130 is rotated in a second direction (e.g. a counterclockwise direction).
Please refer to FIG. 1B again. In the time interval I, the frequencies of the two photoelectronic signals A and B are gradually increased, and the phase of the photoelectronic signal B leads the phase of the photoelectronic signal A by 90 degrees. In other words, the servo motor 130 is rotated in the first direction, and the rotation speed of the servo motor 130 is gradually increased. In the time interval II, the frequencies of the two photoelectronic signals A and B are gradually decreased, and the phase of the photoelectronic signal B leads the phase of the photoelectronic signal A by 90 degrees. In other words, the servo motor 130 is rotated in the first direction, and the rotation speed of the servo motor 130 is gradually decreased until the rotation of the servo motor 130 is stopped.
In the time interval III, the frequencies of the two photoelectronic signals A and B are gradually increased, and the phase of the photoelectronic signal A leads the phase of the photoelectronic signal B by 90 degrees. In other words, the servo motor 130 is rotated in the second direction, and the rotation speed of the servo motor 130 is gradually increased. In the time interval IV, the frequencies of the two photoelectronic signals A and B are gradually decreased, and the phase of the photoelectronic signal A leads the phase of the photoelectronic signal B by 90 degrees. In other words, the servo motor 130 is rotated in the second direction, and the rotation speed of the servo motor 130 is gradually decreased until the rotation of the servo motor 130 is stopped.
From the above discussions, during acceleration or deceleration of the servo motor 130, the frequencies and phases of the two photoelectronic signals A and B are subjected to changes. In other words, the two photoelectronic signals A and B are time variant signals. However, during photoelectric conversion, the two photoelectronic signals A and B may be adversely affected by temperature or environmental factors. For example, the two photoelectronic signals A and B may be suffered from DC offsets or amplitude attenuation, or the phase difference between the two photoelectronic signals A and B fails to be maintained at 90 degrees. Under this circumstance, the position and the rotation speed of the servo motor 130 fail to be accurately acquired according to the feedback pulse.